1282-37-7

Basic information

• Product Name: FERROCENIUM TETRAFLUOROBORATE
• Synonyms: FERROCENIUM TETRAFLUOROBORATE;FERROCENIUM TETRAFLUOROBORATE, TECH.;Ferrocenium tetrafluoroborate technical grade;Bis(cyclopentadienyl)iron tetrafluoroborate;Dicyclopentadienyliron fluoborate;Dicyclopentadienyliron tetrafluoroborate
• CAS NO: 1282-37-7
• Molecular Formula: BF₄*C₁₀H₁₀Fe
• Molecular Weight: 272.84
• Product Categories: Catalysis and Inorganic Chemistry;Chemical Synthesis
• Mol File: 1282-37-7.mol
• Article Data: 7

Chemical Properties

• Melting Point: 178 °C (dec.)(lit.)
• Boiling Point: Vapour density:
• Appearance: solid
• Stability: Stable, but moisture sensitive. Incompatible with strong oxidizing agents.
• CAS DataBase Reference: FERROCENIUM TETRAFLUOROBORATE(CAS DataBase Reference)
• NIST Chemistry Reference: FERROCENIUM TETRAFLUOROBORATE(1282-37-7)
• EPA Substance Registry System: FERROCENIUM TETRAFLUOROBORATE(1282-37-7)

Safety Data

• Hazard Codes: C
• Statements: 34
• Safety Statements: 26-36/37/39-45
• RIDADR: UN 3261 8/PG 2
• WGK Germany: 3
• Hazardous Substances Data: 1282-37-7(Hazardous Substances Data)

Usage

Chemical Properties

solid

Uses

Ferrocenium Tetrafluoroborate can be used for diagnostic and radiotherapeutic composition.

InChI:InChI=1/2C5H.BF4.Fe/c2*1-2-4-5-3-1;2-1(3,4)5;/h2*1H;;/q2*-5;-1;

Synthetic route

tetrafluoroboric acid diethyl ether (67969-82-8) + ferrocene (102-54-5) → ferrocenium(III) tetrafluoroborate (1282-37-7)

Conditions	Yield
In nitromethane; dichloromethane addn. of nitromethane to Fe complex soln. (CH2Cl2) under Ar; addn. of HBF4*Et2O in two portions; stirring, 1h; evapn.; extn. (pentane); stirring with ether; dissoln. in heated CH3CN; filtration through Na2SO4 (free from water) in ether; pptn.; drying (high vac.); elem. anal.;	95%

silver tetrafluoroborate (14104-20-2) + ferrocene (102-54-5) → ferrocenium(III) tetrafluoroborate (1282-37-7)

Conditions	Yield
In tetrahydrofuran byproducts: Ag; (Ar or N2), ferrocene in THF treated with 0.8 equiv. of AgBF4 in THF at -78°C; filtered, evapd.(vac.), extd.(CH2Cl2), crystd. at room temp.;	87%

tetrafluoroboric acid + ferrocene (102-54-5) → ferrocenium(III) tetrafluoroborate (1282-37-7)

Conditions	Yield
In diethyl ether; nitromethane; dichloromethane 12 h;; removing solvent; extn. with petroleum ether 4 times (40-60°C); stirring with ether; filtn.; drying; elem.anal.;;	78%
With p-benzoquinone In diethyl ether; water

ferrocene (102-54-5) → ferrocenium(III) tetrafluoroborate (1282-37-7)

Conditions	Yield
With boron trifluoride diethyl etherate; oxygen In not given passing O2 trough soln. of ferrocene and boron trifluoride etherate;;	45%
With O2; boron trifluoride etherate In not given passing O2 trough soln. of ferrocene and boron trifluoride etherate;;	45%
With p-benzoquinone In benzene addn. of BF3 or boron trifluoride etherate soln. of ferrocene and p-benzoquinone in benzene;;
With BF3 or boron trifluoride etherate; p-benzoquinone In benzene addn. of BF3 or boron trifluoride etherate soln. of ferrocene and p-benzoquinone in benzene;;

ferrocene (102-54-5) + boron trifluoride (7637-07-2) → ferrocenium(III) tetrafluoroborate (1282-37-7)

Conditions	Yield
With oxygen In diethyl ether byproducts: O2(2-); presence of BF3;;
With O2 In diethyl ether byproducts: O2(2-); presence of BF3;;

ferrocene (102-54-5) + tetrabutylammonium tetrafluoroborate (429-42-5) → ferrocenium(III) tetrafluoroborate (1282-37-7)

Conditions	Yield
In acetonitrile Electrochem. Process; Ar atmosphere; oxidn. (Pt mesh, 48.0 C); removal of MeCN (vac., room temp.), no further purification;

sodium tetrafluoroborate (13755-29-8) + ferricinium chloroferrate → ferrocenium(III) tetrafluoroborate (1282-37-7)

Conditions	Yield
In water cooling to -5°C, filtration, washing with water, then with THF, drying in vac.;

ferrocene (102-54-5) + nitrosonium tetrafluoroborate → ferrocenium(III) tetrafluoroborate (1282-37-7)

Conditions	Yield
In dichloromethane prepn. by oxidn. of Cp2Fe with nitrosonium tetrafluoroborate;
In dichloromethane

cyclopentadienyl iron(II) dicarbonyl dimer (38117-54-3) + ferrocenium(III) tetrafluoroborate (1282-37-7) + dimethylsulfide (75-18-3) → {Fe(CO)2(Me2S)(η5-C5H5)}BF4

Conditions	Yield
In dichloromethane byproducts: (C5H5)2Fe; addn. of ferricinium compd. to Fe complex soln. in the presence of SMe2under Ar; filtration; evapn. to 1/3 the volume; addn. of ether/pentane 3/1); pptn.; thoroughly washing (ether, pentane); drying (high vac.);	99%

ferrocenium(III) tetrafluoroborate (1282-37-7) + dicarbonyl(η5-cyclopentadienyl)(η1-7-methoxy-1-cycloheptenyl)iron (95865-41-1) → ferrocene (102-54-5) + dicarbonyl(η5-cyclopentadienyl)(η1-7-methoxycycloheptene-1-carbonyl)iron (95865-47-7)

Conditions	Yield
With `CO In dichloromethane Addn. of Cp2FeBF4 to iron-compd. (methylene chloride, 55 psiCO, 1h).; Removal of solvent (vacuo), elution with hexane (alumina column) gives yellow band of ferrocene, elution with CH2Cl2 gives yellow band of dicarbonyl complex, elem. anal.;	A n/a B 99%

ferrocenium(III) tetrafluoroborate (1282-37-7) + (Pd6(C7H8)4(CH3CN)4)(2+)*2BF4(1-)=(Pd6(C7H8)4(CH3CN)4)(BF4)2 → [Pd3(μ3-cycloheptatriene)2(acetonitrile)3][BF4]2

Conditions	Yield
In acetonitrile room temp.;	99%

ferrocenium(III) tetrafluoroborate (1282-37-7) + [Pd6(μ3-C7H7)4(CH3CN)4][BF4]2 → [Pd3(μ3-C7H7)2(CH3CN)3][BF4]2

Conditions	Yield
In [D3]acetonitrile	99%

ferrocenium(III) tetrafluoroborate (1282-37-7) + (Pd6(C7H7C(CH3)3)4(CH3CN)4)(2+)*2BF4(1-)=(Pd6(C7H7C(CH3)3)4(CH3CN)4)(BF4)2 → [Pd3(μ3-C7H7-t-Bu)2(CH3CN)3][BF4]2

Conditions	Yield
In acetonitrile room temp.;	99%

ferrocenium(III) tetrafluoroborate (1282-37-7) + (Cp2Ti)3(μ3-HATNPh6) → [(Cp2Ti)3(μ3-HATNPh6)][BF4]3

Conditions	Yield
In tetrahydrofuran; hexane at 20℃; Schlenk technique; Glovebox; Inert atmosphere;	99%

cyclopentadienyl iron(II) dicarbonyl dimer (38117-54-3) + ferrocenium(III) tetrafluoroborate (1282-37-7) + dimethylselenide (593-79-3) → {C5H5Fe(CO)2(Se(CH3)2)}(1+)*BF4(1-)={C5H5Fe(CO)2(Se(CH3)2)}BF4

Conditions	Yield
In dichloromethane byproducts: (C5H5)2Fe; addn. of ferricinium compd. to Fe complex soln. in the presence of SeMe2 under Ar; filtration; evapn. to 1/3 the volume; addn. of ether/pentane3/1); pptn.; thoroughly washing (ether, pentane); drying (high vac.);	98%

cyclopentadienyl iron(II) dicarbonyl dimer (38117-54-3) + ferrocenium(III) tetrafluoroborate (1282-37-7) → [dicarbonyl(η5-cyclopentadienyl)iron(II)(PPh3)]BF4

Conditions	Yield
With triphenylphosphine In dichloromethane during intensive stirring addn. of ((C5H5)2Fe)BF4 to mixture of P(C6H5)3 and ((C5H5)Fe(CO)2)2;; crystn. from CH2Cl2/ether; drying; elem.anal.;;	98%
With triphenylphosphine In dichloromethane byproducts: (C5H5)2Fe; addn. of ferricinium compd. to Fe complex soln. in the presence of PPh3under Ar; filtration; evapn. to 1/3 the volume; addn. of ether/pentane 3/1); pptn.; thoroughly washing (ether, pentane); drying (high vac.);	97%

ferrocenium(III) tetrafluoroborate (1282-37-7) + Fe((phenylphosphino)tetramethylcyclopentadienyl) → [1,1'-bis(diphenylphosphino)octamethylferrocenium]BF4 (459790-88-6)

Conditions	Yield
In dichloromethane under Ar, soln. was stirred for 1 h; solvent was removed, ppt. was filtered off, washed with benzene repeatedly, dried in vac.; elem. anal.;	98%

ferrocenium(III) tetrafluoroborate (1282-37-7) + (OC)3Fe((CH2)3S2)Ni(Ph2P(CH2)2PPh2) (877034-21-4, 877034-22-5) + iodine (7553-56-2) → [(1,2-bis(diphenylphosphino)ethane)Ni(2,2-dimethyl-1,3-propanedithiolate)IFe(CO)3]BF4

Conditions	Yield
Stage #1: ferrocenium(III) tetrafluoroborate; (OC)3Fe((CH2)3S2)Ni(Ph2P(CH2)2PPh2) In dichloromethane for 0.0166667h; Stage #2: iodine In dichloromethane for 0.00833333h;	98%

ferrocenium(III) tetrafluoroborate (1282-37-7) + (OC)3Fe((CH2)3S2)Ni(Ph2P(CH2)2PPh2) (877034-21-4, 877034-22-5) + bromine (7726-95-6) → [(1,2-bis(diphenylphosphino)ethane)Ni(2,2-dimethyl-1,3-propanedithiolate)BrFe(CO)3]BF4

Conditions	Yield
Stage #1: ferrocenium(III) tetrafluoroborate; (OC)3Fe((CH2)3S2)Ni(Ph2P(CH2)2PPh2) In dichloromethane for 0.0166667h; Stage #2: bromine In dichloromethane for 0.00833333h;	98%

N-Bromosuccinimide (128-08-5) + ferrocenium(III) tetrafluoroborate (1282-37-7) + (OC)3Fe((CH2)3S2)Ni(Ph2P(CH2)2PPh2) (877034-21-4, 877034-22-5) → [(1,2-bis(diphenylphosphino)ethane)Ni(2,2-dimethyl-1,3-propanedithiolate)BrFe(CO)3]BF4

Conditions	Yield
Stage #1: ferrocenium(III) tetrafluoroborate; (OC)3Fe((CH2)3S2)Ni(Ph2P(CH2)2PPh2) In dichloromethane for 0.0166667h; Stage #2: N-Bromosuccinimide In dichloromethane for 0.00833333h;	98%

ferrocenium(III) tetrafluoroborate (1282-37-7) + (Cp2Ti)3(μ3-HATNPh6) → [(Cp2Ti)3(μ3-HATNPh6)]BF4

Conditions	Yield
In hexane at 20℃; Schlenk technique; Glovebox; Inert atmosphere;	98%

bis(cyclopentadienyl)chromium (1271-24-5) + ferrocenium(III) tetrafluoroborate (1282-37-7) → [Cr(C5H5)2](1+)*BF4(1-)=[Cr(C5H5)2]BF4 (106453-51-4)

Conditions	Yield
In dichloromethane byproducts: (C5H5)2Fe; addn. of ferricenium compd. to Cr complex soln. under Ar; filtration; evapn. to 1/5 of the volume; addn. of ether; pptn.; recrystn. from CH3CN/ether (1/1); drying (high vac.); elem. anal.;	97%

ferrocenium(III) tetrafluoroborate (1282-37-7) + [Fe2(η5-C5H5)2(μ-CO)2(CO)(CyPH2)] (869802-98-2) → [Fe2(η5-C5H5)2(μ-CO)(CO)2(μ-PCyH)]BF4 (869803-01-0)

Conditions	Yield
In dichloromethane (N2); std. Schlenk technique; solid (C5H5)2FeBF4 was added to stirred soln. of Fe-P complex in CH2Cl2 at 273 K; stirred for 10 min; evapd. (vac.); washed (petroleum ether); elem. anal.;	97%
In not given byproducts: H2;

ferrocenium(III) tetrafluoroborate (1282-37-7) + [Co(TC-5,5)] (169232-86-4) → [Co(TC-5,5)](BF4)

Conditions	Yield
In dichloromethane Inert atmosphere; Glovebox;	97%

ferrocenium(III) tetrafluoroborate (1282-37-7) + hexarutheniumhexadecacarbonylcarbido di(bis(triphenylphosphine)iminium) → carbido heptadecacarbonyl hexaruthenium

Conditions	Yield
With carbon monoxide In not given dropwise addn. of CO and Ru-cluster soln. (CH2Cl2 or MeCN) to soln. of Cp2FeBF4 (N2 purge, room temp.); IR monitoring;	96%

cyclopentadienyl iron(II) dicarbonyl dimer (38117-54-3) + ferrocenium(III) tetrafluoroborate (1282-37-7) → cyclopentadienyldicarbonyl(tetrahydrofuran)iron(II) tetrafluoroborate

Conditions	Yield
With tetrahydrofuran In dichloromethane byproducts: (C5H5)2Fe; addn. of ferricinium compd. to Fe complex soln. in the presence of THF under Ar; filtration; evapn. to 1/3 the volume; addn. of ether/pentane 3/1); pptn.; thoroughly washing (ether, pentane); drying (high vac.);	96%

nickelocene (1271-28-9) + ferrocenium(III) tetrafluoroborate (1282-37-7) → (C5H5)2Fe*(C5H5)2Ni(2+)*2BF4(1-) = {(C5H5)4FeNi}(BF4)2

Conditions	Yield
In dichloromethane byproducts: Fe(C5H5)2; mixed, standed (1 min); ppt. washed (CH2Cl2), dried (vac.); elem. anal.;	96%

nickelocene (1271-28-9) + ferrocenium(III) tetrafluoroborate (1282-37-7) → {nickelocenium} tetrafluoroborate

Conditions	Yield
In dichloromethane byproducts: (C5H5)2Fe; addn. of ferricenium compd. to Ni complex soln. under Ar; filtration; evapn. to 1/5 of the volume; addn. of ether; pptn.; recrystn. from CH3CN/ether (1/1); drying (high vac.); elem. anal.;	96%
In dichloromethane dissolving of nickelocene in a minimum of CH2Cl2, addn. of the ferrocenium tetrafluoroborate under Ar, standing for 72 h; filtn., concn., washing with cold CH2Cl2 and Et2O and placing in vac. for several hours;

dicobalt octacarbonyl (15226-74-1, 61091-28-9, 61117-58-6) + ferrocenium(III) tetrafluoroborate (1282-37-7) → Co(CO)3(P(C6H5)3)2(1+)*BF4(1-)=Co(CO)3(P(C6H5)3)2BF4 (100852-19-5, 118400-45-6)

Conditions	Yield
With triphenylphosphine In dichloromethane addn. of ((C5H5)Fe)BF4 to mixture of P(C6H5)3 and Co(CO)8; addn. of ether to sol.;; elem.anal.;;	96%

cyclopentadienyl iron(II) dicarbonyl dimer (38117-54-3) + ferrocenium(III) tetrafluoroborate (1282-37-7) + tri-tert-butyl phosphine (13716-12-6) → [C5H5Fe(CO)2P(C(CH3)3)3](1+)*BF4(1-)=[C5H5Fe(CO)2P(C(CH3)3)3]BF4 (106453-55-8)

Conditions	Yield
In dichloromethane byproducts: (C5H5)2Fe; addn. of ferricinium compd. to Fe complex soln. in the presence of phosphane under Ar; filtration; evapn. to 1/3 the volume; addn. of ether/pentane (3/1); pptn.; thoroughly washing (ether, pentane); drying (high vac.); elem. anal.;	96%

dicobalt octacarbonyl (15226-74-1, 61091-28-9, 61117-58-6) + ferrocenium(III) tetrafluoroborate (1282-37-7) → [Co(CO)3(P(C6H5)3)2](1+)*BF4(1-)=[Co(CO)3(P(C6H5)3)2]BF4 (100852-19-5, 118400-45-6)

Conditions	Yield
With triphenylphosphine In dichloromethane byproducts: (C5H5)2Fe; addn. of ferricinium compd. to Co complex soln. in the presence of PPh3under Ar; filtration; evapn. to 1/3 the volume; addn. of ether/pentane (3/1); pptn.; thoroughly washing (ether, pentane); drying (high vac.); elem. anal.;	96%

ferrocenium(III) tetrafluoroborate (1282-37-7) + bis(benzene)chromium(0) (1271-54-1) → [Cr(C6H6)2](1+)*BF4(1-)=[Cr(C6H6)2]BF4 (11077-48-8)

Conditions	Yield
In dichloromethane byproducts: (C5H5)2Fe; addn. of ferricenium compd. to Cr complex soln. under Ar; filtration; evapn. to 1/5 of the volume; addn. of ether; pptn.; recrystn. from CH3CN/ether (1/1); drying (high vac.); elem. anal.;	96%

ferrocenium(III) tetrafluoroborate (1282-37-7) + Cp(*)Fe(as-indacene)FeCp(*) → [Cp(*)Fe(as-indacene)FeCp(*)](.+)[BF4(1-)] (129500-53-4)

Conditions	Yield
(N2); elem. anal.;	96%

ferrocenium(III) tetrafluoroborate (1282-37-7) + [Fe2(η5-C5H5)2(μ-CO)2(CO)(PH2Ph)] (869802-99-3) → [Fe2(η5-C5H5)2(μ-CO)(CO)2(μ-PPhH)]BF4 (869881-53-8)

Conditions	Yield
In dichloromethane (N2); std. Schlenk technique; solid (C5H5)2FeBF4 was added to stirred soln. of Fe-P complex in CH2Cl2 at 273 K; stirred for 10 min; evapd. (vac.); washed (petroleum ether); elem. anal.;	96%
In not given byproducts: H2;

ferrocenium(III) tetrafluoroborate (1282-37-7) + (triphenylphosphane)gold(I) tert-butylthiolate (122711-31-3) → ferrocene (102-54-5) + tetrakis(triphenylphosphane)bis(tert-butylthiolate)tetragold(I) tetrafluoroborate

Conditions	Yield
In dichloromethane byproducts: (SC(CH3)3)2; N2, Fe:Au=0.136:0.276 molar ratio, Fe compd. added to a soln. of Au compd., stirred for 30 min; solvent evapd., residue washed (diethyl ether, benzene), dried (vac.), recrystd. (CH2Cl2, -5°C), org. phases evapd. to dryness, ferrocenesublimied (40°C, high vac.); elem. anal.;	A n/a B 96%

ferrocenium(III) tetrafluoroborate (1282-37-7) + dichloromethane (75-09-2) + (OC)3Fe((CH2)3S2)Ni(Ph2P(CH2)2PPh2) (877034-21-4, 877034-22-5) + P(p-CH3OC6H4)3 (855-38-9) → [(dppe)Ni(propaneditiolate)Fe(CO)2(P(p-C6H4OMe)3)]BF4*0.25CH2Cl2

Conditions	Yield
With pentane In dichloromethane MBraun glovebox, under N2; soln. of NiFe compd. and FcBF4 in CH2Cl2, rapid stirring for 1 min, above soln. added to phosphine in CH2Cl2, 0.5 min, pentane added (-28°C, then 1 h at this temp.); filtered, washed (pentane, -28°C), dried; elem. anal.;	96%

ferrocenium(III) tetrafluoroborate (1282-37-7) + [Fe(CO)2(2-[(di-tert-butylphosphino)methyl]-6-[1-(2,4,6-mesitylimino)ethyl]pyridine)] → [Fe(CO)2(2-[(di-tert-butylphosphino)methyl]-6-[1-(2,4,6-mesitylimino)ethyl]pyridine)](BF4)

Conditions	Yield
In tetrahydrofuran at 20℃; for 3h; Inert atmosphere;	96%

ferrocenium(III) tetrafluoroborate (1282-37-7) + [Cp2Ni2(ethanethiol)2] → [Cp2Ni2(ethanethiol)2]BF4

Conditions	Yield
In dichloromethane	96%

Upstream product

• 102-54-5 ferrocene 
• 429-42-5 tetrabutylammonium tetrafluoroborate 
• 14104-20-2 silver tetrafluoroborate 
• 67969-82-8 tetrafluoroboric acid diethyl ether 
• 13755-29-8 sodium tetrafluoroborate 
• 7637-07-2 boron trifluoride 

Downstream Products

• 136005-96-4 {Ru(PPh₃)2(CO)3(Cl)}{BF₄} 
• 128205-84-5 {(η4-2-(p-fluorophenyl)-1,3-butadiene)cobalt(carbonyl)3} tetrafluoroborate 
• 128205-82-3 {(η4-2-phenyl-1,3-butadiene)cobalt(carbonyl)3} tetrafluoroborate 
• 33270-47-2 (C₅H₅)Mo(CO)2((C₆H₅)3P)HgCl 
• 130036-39-4 (C₅H₅)Mo(CO)2((C₆H₅)3P)(CH₃CN)⁽¹⁺⁾*BF₄^(1-)={(C₅H₅)Mo(CO)2((C₆H₅)3P)(CH₃CN)}{BF₄} 
• 78833-68-8 (C₅H₅)Mo(CO)2((C₆H₅)3P)2⁽¹⁺⁾*BF₄^(1-)={(C₅H₅)Mo(CO)2((C₆H₅)3P)2}{BF₄} 
• 102211-01-8 Fe₂(CO)7(P(C₆H₁₁)2) 
• 102-54-5 ferrocene 

Relevant articles and documents

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Redox behavior of nickel acylate complexes

The cyclic voltammetry and bulk oxidation with ferrocenium ion and molecular iodine of three nickel complexes were studied. The results are most consistent with both Cp2Fe+ and I2 acting as single-electron oxidants.

Solvent Dynamical Effects in Electron Transfer: Comparisons of Self-Exchange Kinetics for Cobaltocenium-Cobaltocene and Related Redox Couples with Theoretical Predictions

Rate constants, Kex, and activation parameters for the self-exchange of cobaltocenium-cobaltocene Cp2Co+/0, and the decamethyl derivative Cp'2Co+/0, in 13 organic solvents have been evaluated by using the proton NMR line-broadening technique with the objective of probing the influence of solvent dynamics upon the electron-transfer kinetics.Together with some corresponding measurements reported earlier for ferrocenium-ferrocene Cp2Fe+/0, additional measurements for the decamethyl derivative, Cp'2Fe+/0, and with corresponding data for Cp2Co+/0 electrochemical exchange, these results enable a systematic comparative examination to be made of the effects of solvent dielectric relaxation on the barrier-crossing frequency for such simple outer-sphere reactions.For the facile Cp'2Co+/0 couple the solvent dependence of the observed frequency factors, νn(obsd), extracted from the kex values by correcting for the solvent-dependent barrier height, ΔG*, is in approximate accordance with the relative frequency factors νos(calcd), predicted from the continuum model of overdamped solvent relaxation.The subunity (ca. 0.7-0.8) slope of the logarithmic νn(obsd) - ??os(calcd) plot for Cp'2C0+/0 self-exchange is consistent with a recent theoretical prediction of the combined effect of overdamped solvent motion and reactant vibrations (ref 2g).In a given solvent, the sequence of kex values is Cp'2Co+/0 > Cp2Co+/0 ca.Cp'2Fe+/0 > Cp2Fe+/0, with Cp2Co+/0 and Cp2Fe+/0 being about 10- and 100-folds lower, respectively, than Cp'2Co+/0 self-exchange.While these reactivity differences can be traced to variations in donor-acceptor orbital overlap, the solvent dependencies of kex for Cp2Co+/0 and Cp2Fe+/0 electron exchange nevertheless exhibit a strong influence from overdamped solvent relaxation.Marked deviations from the dielectric continuum predictions are seen, however, in several solvents.Thus the barrier-crossing frequencies in propylene carbonate, N-methylformamide, and especially methanol are substantially (4-100-fold) larger than expected from νos(calcd), implicating the presence of surprisingly rapid relaxation modes in these solvents.The solvent-dependent activation parameters also differ significantly from the expectations of conventional theoretical modes.